FIG. 1 shows a conventional optical wavelength multiplexing transmission system. This system is an example of two-wavelength-multiplexing optical signal transmission system, where each of up signal light wavelengths .lambda.1, .lambda.3 and down signal light wavelengths .lambda.2, .lambda.4 is multiplexed and transmitted.
In an up optical transmission end office 1a, signal lights 6 with wavelengths .lambda.1, .lambda.3 output from a signal light source 5 are multiplexed by an optical multiplexer 7 and then amplified by an optical amplifier 8, thereafter being transmitted through an optical fiber transmission line 3 and an optical amplifier repeater 4 to an up optical reception end office 2a. In the up optical reception end office 2a, the signal lights 6 are again amplified by an optical amplifier 8, being divided by an optical divider 9, being subject to the wavelength selection by optical filters 10 which correspond to the respective wavelengths .lambda.1, .lambda.3, and being received by optical receivers 11. The down optical transmission system where signal lights with wavelengths .lambda.2, .lambda.4 are transmitted has a like composition. In general, as the optical multiplexer 7 and optical divider 9, a wavelength multiplexing coupler, a fiber coupler or the like is used.
In the optical wavelength multiplexing transmission system, the signal lights with the wavelengths .lambda.1, .lambda.3 or .lambda.2, .lambda.4 which are output from the signal light sources 5 will have different gains and losses to the respective wavelengths due to the optical amplifier 8, optical fiber transmission line 3, optical coupler 7, 9 for coupling or dividing etc. This is because the gain or loss property in the above components composing the optical transmission line depends on wavelengths, particularly the optical amplifier 8 having a significant wavelength-dependency for gain. Here, erbium-doped optical fiber amplifiers, which are at present most generally used for the optical communication, also have the wavelength-dependency for gain. Therefore, research and development are being made such that they have a constant gain, i.e., a leveled gain, to different wavelengths of signal light to be suitable for the wavelength multiplexing transmission. Other than fiber amplifiers such as the erbium-doped optical amplifier, a semiconductor amplifier to which a semiconductor laser is applied can be used, but it also has the wavelength-dependency for gain.
Meanwhile, in the respective signal light sources 5 of the transmission end office, feedback control(output power control) is in general conducted to control the optical power(optical electric power) of the output signal light to be always constant while monitoring a part of the output signal light. For example, there is a method that a light-receiving photodiode(PD) detects the power of back emitting light of a laser diode(LD) as a signal light source to control the drive current of the laser diode.
At present, as described above, improvements of the wavelength property in such optical amplifiers and control techniques of the optical power of signal light sources are used in the development of optical wavelength multiplexing transmission system.
However, when an optical wavelength multiplexing transmission system is actually constructed and operated, the property change with time in the optical amplifier, optical fiber transmission line, optical couplers for multiplexing and dividing etc. may cause the change of gain or loss in signal lights with different wavelengths. Particularly in the optical amplifier, since the wavelength property for gain changes depending on the power(electric power) of the input signal light, the power of the is signal light input to the optical amplifier is reduced due to the increase in loss of the optical fiber transmission line with time, therefore damaging the levelness of gain of optical amplifier. Thus, in the signal light with a wavelength greatly affected by this, the power(electric power) of the signal light may be weakened when received by the optical reception end office, therefore not giving a sufficient S/N ratio.
This situation is explained with reference to (a) to (f) in FIG. 2. The situation of the up optical transmission line is shown by (a), (b) and (c) in FIG. 2, the situation of the down optical transmission line is shown by (d), (e) and (f) in FIG. 2. FIG. 2 shows the case that the up optical transmission line is normally operated and the wavelength property in the down optical transmission line is not normal. In FIG. 2, (a) and (d) show outputs of the signal lights 6 emitted from the signal light sources 5 of up and down optical transmission end offices 1a, 1b, respectively, (b) and (e) show the wavelength properties of up and down optical transmission systems, respectively, (c) and (f) show inputs of the signal lights 6 to the up and down optical reception end offices 2a, 2b, respectively.
If the wavelength property of the optical transmission line is, as shown in FIG. 2(e), not normal, the power of the signal light entering to the down optical reception end office 2b is affected by this, and particularly in the signal light(with the wavelength .lambda.2 shown in FIG. 2(f)) having much loss in the corresponding optical transmission line the power of receiving light is reduced, therefore deteriorating the S/N ratio.